The invention relates to a substrate, in particular for the EUV micro-lithography, the manufacture of such a substrate, as well as to the use of such a substrate for mirrors and/or masks, or mask blanks, respectively, in the EUV micro-lithography.
In the manufacture of integrated circuits the tendency is going towards chips having smaller and smaller structures. Therefore, for producing such chips, lithography systems are necessary that can operate at exposure wavelengths that are becoming smaller and smaller. Currently, wavelengths of 248, 193 and 157 nm are used. For the future, for such lithography systems the application of electromagnetic radiation in the so-called extreme UW region (EUV), in particular in the range of 11 to 14 nm, has been proposed. In this range it will become necessary to leave the transmissive systems common in the prior art and to proceed to reflective systems having reflective optical elements and masks. Up to now, for such reflective systems only few substrates for masks and also for mirrors as well as the related optics that can fulfill the extreme requirements of this technique have been described.
From U.S. Pat. No. 6,159,643 a reflective mask is known that consists of a structured layer and a substrate, wherein the substrate consists of a base layer of a material having a small coefficient of thermal expansion, such as ULE® having a coefficient of expansion of about 0.02 ppm/K, and that is combined with a cover layer of silicon, for instance by an adhesive.
However, with such a substrate the requirements with respect to shape precision and surface roughness of the substrate cannot be met. For instance, for the masks a shape precision of less than 50 nm PV (peak to valley, PV) is required.
Also optical systems based on Zerodur® substrates are known in the art. It was found however in this regard that, although these can be polished with known polishing methods to a surface roughness of 0.1 to 0.3 nm, this surface roughness will be lost during a commonly following treatment step by ion beam figuring (IBF). The surface roughness during this treatment increases by a factor of 2 to 5, if no additional precautions are taken. Since however a surface roughness of less than 0.2 nm rms is desired for substrates for the EUV lithography, the application of such substrates for optical systems suitable for production requires tremendous effort.
Therefore, it is a first object of the invention to provide a process of manufacturing a substrate that is particularly suited for the EUV micro-lithography.
It is a second object of the invention to provide a process of manufacturing a substrate that is simple and cost effective.
It is a third object of the invention to provide a process of manufacturing a substrate for use in the EUV micro-lithography ensuring an extremely high shape precision and low surface roughness.
It is a further object of the invention to provide a component for use in the EUV micro-lithography, such as a mask blank, a mask, a structured mask or a mirror.